The Agriculture Department last night took the unusual step of insisting that U.S. farmers refrain from planting a popular variety of long-grain rice because preliminary tests showed that its seed stock may be contaminated with a variety of gene-altered rice not approved for marketing in the United States. The announcement marks the third time in six months that U.S. rice has been found to be inexplicably contaminated with engineered traits, and it comes just weeks before the spring planting season.